On Wed, Sep 25, 2019 at 06:03:20PM +0100, Juan Casanova wrote:
why isn't traverse just defined as
traverse_alt :: (Traversable t, Applicative f) => t (f a) -> f (t a) traverse_alt = traverse id
and let fmap deal with the mapping of the function? Of course this wouldn't be the implementation, it would be the other way around. Instances of Traversable would implement traverse_alt, and then whenever I wanted to do what traverse currently does, I would just do: traverse_alt (fmap f inputs). What is there to gain by including the mapping into the traversal *in the implementation of traverse itself*?
Your `traverse_alt` is called `sequence`: https://hackage.haskell.org/package/base-4.12.0.0/docs/src/Data.Traversable.... They're both useful and you can define each in terms of the other. I'd say I use traverse about ten times more than sequence. It's basically equivalent to `mapM`, i.e. "map with an effect". `fmap` followed by `sequence` tends to read a bit clumsily. Tom